A two step process, positive/negative direct current (de) biasing during the substrate pretreatment stage without biasing during the subsequent deposition stage, was used in this study. Microstructure of positive/negative dc bias-enhanced diamond deposition by microwave plasma chemical vapor deposition on silicon substrate was investigated by Raman spectroscopy, scanning electron microscopy and high resolution transmission electron microscopy (HRTEM). In positive de biasing pretreatment, high methane concentration (25.9% CH(4) in H(2)) was used to enhance diamond nucleation and resulted in the co-deposition of diamond nuclei and amorphous carbon phase. Continuous polycrystalline diamond film was observed after positive dc biasing for 5 min and growth deposition. From HRTEM cross-section analysis, it is observed that an amorphous carbon interlayer was obtained between diamond-silicon interface after pretreatment and deposition processes. The process for negative dc biasing enhanced diamond deposition was also studied and discussed. HRTEM result shows that neither SiC nor other intermediate layer is revealed between the diamond and silicon after negative dc biasing and growth processes. (c) 2007 Elsevier B.V. All rights reserved.